# SUMO E3 ligase MUL1 inhibits lymph node metastasis of bladder cancer by mediating mitochondrial HSPA9 translocation

## TL;DR

This study shows how the SUMO E3 ligase MUL1 prevents bladder cancer from spreading to lymph nodes by controlling mitochondrial protein HSPA9.

## Contribution

The novel finding is that MUL1 inhibits bladder cancer metastasis by SUMOylating HSPA9 and regulating downstream pathways.

## Key findings

- MUL1 is downregulated in metastatic bladder cancer tissues and is associated with better patient outcomes.
- MUL1 SUMOylates HSPA9 at K612, promoting its translocation to the nucleus and interaction with SUZ12 and EZH2.
- MUL1 inhibits bladder cancer cell metastasis and proliferation through HSPA9-dependent degradation of SUZ12 and EZH2.

## Abstract

Lymph node (LN) metastasis is the dominant cause of death in bladder cancer (BCa) patients, but the underlying mechanism remains largely unknown. In recent years, accumulating studies have confirmed that bidirectional mitochondria-nucleus communication is essential for sustaining multiple function of mitochondria. However, little has been studied regarding whether and how the translocation of mitochondrial proteins is involved in LN metastasis. In this study, we first identified that the SUMO E3 ligase MUL1 was significantly downregulated in LN-metastatic BCa tissues and correlated with a good prognosis. Mechanistically, MUL1 SUMOylated HSPA9 at the K612 residue, leading to HSPA9 export from mitochondria and interaction with SUZ12 and in the nucleus. Consequently, MUL1 induced the ubiquitination-mediated degradation of SUZ12 and EZH2 and induced downstream STAT3 pathway inhibition in a HSPA9-dependent manner. Importantly, mutation of HSPA9 SUMO-conjugation motifs limited the translocation of mitochondrial HSPA9 and blocked the HSPA9-SUZ12 and HSPA9-EZH2 interactions. With mutation of the HSPA9 K612 site, the suppressive role of MUL1 overexpression was lost in BCa cells. Further in vitro and in vivo assays revealed that MUL1 inhibits the metastasis and proliferation of BCa cells. Overall, our study reveals a novel function and molecular mechanism of SUMO E3 ligases in LN metastasis.